Page 176 - Science at the nanoscale

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gain sufficient thermal energy to overcome the energy barrier that
prohibits the electrons from escaping. The higher the tempera-
ture, the higher the number of electrons emitted. Tungsten is
commonly chosen as the filament because it can withstand high
temperature without melting. However, thermionic electron guns
have relatively low brightness.
On the other hand, the field emission electron gun relies on elec-
trons emitted from a sharp tip upon the application of a high elec-
tric field. It does not involve heating of a filament. Instead, when a
high electric field is applied to the tip, electrons from the tip quan-
tum mechanically tunnel through the energy barrier into the vac-
uum. Typically the field emission gun has two anodes. The first
anode (at ∼0–5 kV) serves to extract the electrons from the tip,
while the second anodes (at ∼1–50kV) serves to accelerate the
electrons and this determines the energy of the electrons travel-
ing down the column of the SEM. The field emission electron gun
has a higher brightness.
As the electrons are streaming out from the electron guns, they
form a spray pattern. In order to control the profile of this electron
beam into a finely adjusted focused beam, electromagnetic lenses
are used. When an electron with charge q and velocity ~v travels
~
~
in a region with a magnetic field B, it will experience a force F
given by:
~
~
F = q~v × B
(8.3)
One thing to note is that since the direction of the force is acting
perpendicular to the direction of the velocity, the Lorentz force
acting on the electrons has no effect on the speed of the electron.
The only effect the magnetic field has on the electron is to change ch08
the direction of motion of the electrons.
As shown in Fig. 8.7, the magnetic field profile generated by a
typical electromagnet used in a SEM is highly non-uniform. The
magnetic field of the electromagnetic lens can be considered to
be made up of two independent components, the vertical axial
component (Hz) and the horizontal radial component (Hr). The
radial component causes the electron traveling in the −z direction
to move in a helical manner with respect to the central axis. The
axial component causes the electron to move closer to the central
axis, i.e., the effect of the axial component is to reduce the diameter
of the helical path of the electrons. As a result, the electron beam